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46th Lunar and Planetary Science Conference (2015)
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CHARACTERIZING MARE DEPOSITS IN THE AUSTRALE REGION. S. J. Lawrence1, J. D. Stopar1, B. L.
Jolliff2, M. S. Robinson1, H. Sato1, H. Hiesinger3, B. R. Hawke4, T. A. Giguere4,5 1School of Earth and Space Exploration, Arizona State University, Tempe, AZ ([email protected]) 2Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO 3Institut für Planetologie, Westfälische Wilhelms-Universität Münster,
Germany 4Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 5Integraph Corporation, Honolulu, HI
Introduction: A key goal of the Lunar Reconnaissance Orbiter (LRO) second extended mission (ESM2)
is to investigate volcanic processes at different temporal
and physical scales including the identification and
characterization of ancient (meaning: > 3.9 Ga) volcanic
units. Mare Australe (a loosely-circular collection of
mare basalts centered at approximately 38.9° S, 93° E)
potentially includes ancient volcanic deposits and is a
complex, extensive, and poorly understood volcanic region [1-5].
Background: Whitford-Stark [1] postulated four
periods of mare eruptive activity from the late Nectarian
to the Eratosthenian based on geologic mapping. Absolute model ages for a subset of basalt units in the Australe region range from 3.08-3.91 Ga [6]. Mare Australe
has been proposed to lie within a pre-Nectarian impact
basin [7-10], but does not have a well-defined basin rim
boundary [11], thus the underlying cause of Mare Australe’s circularity is not yet understood. Fundamental information about the early stages of mare formation processes uncoupled from basin structure may be preserved
in the discrete Australe basalt deposits, which each potentially represent one or several disconnected eruptive
events.
Goals: The goals of our study are to: 1) identify and
characterize the discrete basalt deposits in the Australe
region using new LRO data products, 2) identify possible basaltic source vents, and 3) further characterize
mare stratigraphy and evolution of mare sources in the
region. Previously, we reported preliminary work using
new LRO data to understand the distribution of volcanic
landforms and the extent of mare basalt exposures in the
Australe region [4].
In this work, we determine new preliminary absolute
model ages (AMAs) for a representative sampling of
basalts, smooth plains, and cryptomaria in Australe, as
well as characterize the physical parameters and compositions of these units using new data from LRO and
legacy data from other lunar missions.
Methods: This study uses new observations and
data products from the Lunar Reconnaissance Orbiter,
particularly the LRO Wide Angle Camera (WAC). The
WAC is a push-frame camera capturing seven color
bands (321, 360, 415, 566, 604, 643, and 689 nm) with
a 57-km swath width in color mode and a 105-km swath
width in monochrome mode from a 50-km altitude [12].
Primary data products employed in this investigation include the WAC global morphology base map, the
GLD100 global topography dataset [13], and the recent
Hapke parameter photometrically-corrected WAC color
dataset of Sato et al. [14]. The GLD100 (spatial resolution sampling of 100 m and a vertical accuracy of 10m)
is being used for all topographic measurements. The
GLD100 was used to compute the Terrain Ruggedness
Index (TRI), the mean elevation difference between the
central DTM pixel and its surrounding cells, at a pixel
scale of 100 m [15].
New crater size frequency distributions (CSFDs) for
44 surface units in the Australe region were collected
and used to determine absolute model ages (AMAs) following the methods of Hiesinger et al. [16] applied to
the WAC morphology base map. 23 of the count areas
were the same as those used in Hiesinger et al. [6]. All
craters larger than 1km in diameter identified in the
LROC WAC global morphology base map were included in the counts.
Individual landforms in the Australe region are being assessed using LRO Narrow Angle Camera (NAC)
monoscopic observations. Five NAC Digital Terrain
Models (DTMs) with pixel scales of 2m were used to
investigate the topography and surface roughness characteristics of individual mare and highland units in the
region.
Finally, we used legacy datasets, including Clementine FeO and TiO2 maps of the Australe region produced
using the techniques of [17] controlled to the WAC morphology basemap, and Th abundances extracted from
the reduced half-degree Th dataset collected during the
low-altitude portion of the Lunar Prospector mission
[18], to characterize the composition of individual units.
The boundaries of each deposit were based on those
from Hiesinger et al. [6] and refined using the WAC
color data of Sato et al. [14]. Taken collectively, these
data products enable us to analyze the physical parameters and compositions of Australe units as a function of
age.
Results: Model Ages: The new AMAs determined
for this study using LROC data are generally in good
agreement with those previously reported [6]. Count regions included mare basalt exposures, smooth plains
units, and prospective cryptomare. Our new AMAs for
Australe basalt units are between 3.1 Ga and 4.1 Ga,
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Basalt Units
8
7
6
5
4
3
2
1
0
Smooth Plains
2.5
Th (ppm)
Frequency
46th Lunar and Planetary Science Conference (2015)
2
1.5
1
3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2
Absolute Model Age (Ga)
Figure 2. Histogram of Absolute Model Ages determined for
geologic units in the Australe region.
spanning the Late Imbrian to the Pre-Nectarian epochs
(Fig 1.).
Topography: Mean equipotential surface elevations
for the units characterized in this work range from 15m
to –3770m. We discern no meaningful correlation between the AMAs and the elevations of the individual
basalt units.
GLD100 Morphometry: At the 100 m pixel scale of
the GLD100, the TRI values for the basalt units in the
Australe region range from 1.3 to 3.4, and are not correlated with the AMAs.
NAC DTM Morphometry: Using NAC DTMs, we
assessed the morphometry and surface roughness of
three basalt units and a representative highlands unit
near Hanno crater at the 2m/pixel scale of the NAC
DTMs. At this pixel scale, the average TRI values of the
three basalt units (0.14-0.18) are significantly less than
the average highlands TRI (0.27). We are continuing to
investigate the utility of NAC-derived morphometric
parameters such as the TRI in quantitatively identifying
and mapping the extents of cryptomare and/or smooth
plains units. For this purpose, additional NAC geometric stereo observations are being acquired of cryptomaria and other potential volcanic landforms in the
Australe region.
Composition: FeO values for the soils developed on
mare deposits vary between 11 and 15 weight %. TiO2
values vary between 0.7 to 3.8 weight %, compositions
that likely reflect nonmare contamination [e.g., 19]. No
strong correlation between the AMAs and the FeO and
TiO2 content were observed.
Th concentrations for the basalt units are between
1.5 and 2.5 ppm Th (Fig. 2), and are largely indistinguishable from nonmare Th concentrations in the Australe region. Three of the units with AMA values of 3.13.3 Ga have elevated Th values, but all other units investigated in this study (including smooth plains units)
have values between 1.5 and 2 ppm. Thus, there is also
no strong relationship between the model age and Th
abundance.
3
3.5
4
4.5
Model Age (Ga)
Figure 1. Th abundances of geologic units in the Australe region.
Implications: The lack of temporal and spatial correlations between compositions, morphometric parameters, and ages of deposits in the Australe region suggests
that the various basalt units in the Australe region have
tapped discrete but not highly variable mantle source regions, as suggested by Gillis and Jolliff [3]. The relatively low abundances of Th when compared to the PKT
mare units [20] and farside Th anomalies like Dewar
[21] and Compton-Belkovich [22], provides evidence
that the distribution of KREEP elements including Th in
the mantle is discontinuous and nonuniform, but in the
Australe region there appears to have been no significant fluctuation with time.
Acknowledgements: This work is supported by the
LRO mission.
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